Kepler-78b, a planet about 400 light-years away, is like hell on Earth.

Astronomers described it yesterday as the first Earth-size planet that seems to be made of the
same mixture of rock and iron as Earth, and that orbits a star similar to our sun.

But Kepler-78b would not be a pleasant place to visit. It whirls around its parent star,
Kepler-78, at a distance of less than 1 million miles, and its year — the time it takes to complete
one orbit — is just 8 1/2 hours. (By contrast, Earth is 93 million miles from the sun and, of
course, completes its yearly orbit in a little more than 365 days.)

At that close proximity, the surface of Kepler-78b is infernally hot: 3,500 to 5,000 degrees, or
“well above the temperature where rock melts,” said Andrew W. Howard, an astronomer at the
University of Hawaii and the lead author of one of two papers being published in the journal
Nature. “This is probably one of the most hellish planets that have been discovered
yet."

Viewed from the surface of Kepler-78b, its star would cover 80 times more of the sky than the
sun does in Earth’s sky.

“It’s certainly not a habitable planet,” said Francesco Pepe, a professor of astronomy at the
University of Geneva and the lead author of the other
Nature paper.

Kepler-78b is one of more than 150 planets spotted by NASA’s Kepler spacecraft, which noted the
dimming of the starlight when a planet passed in front.